1. Field of the Invention
The present invention provides multilayer ion plated coatings comprising a titanium oxide as well as methods for applying such coatings onto a variety of substrates. In particular, the invention provides ion plated transparent multilayer coatings comprising layers titanium oxide (particularly TiO.sub.x, x.apprxeq.2) and materials of low refractive indices such as SiO.sub.2, Al.sub.2 O.sub.3, MgO etc., articles of manufacture comprising such multilayer coatings, and novel methods for deposition of such multilayer coatings.
2. Background
Optical filters and coatings are passive components whose basic function is to define or improve the performance of optical systems. Applications of optical filters and coatings can be as diverse as anti-glare computer screens, laser devices such as ophthalmic surgical lasers, sighting devices, etc.
Optical coatings have been deposited by Physical Vapor Deposition (PVD) technologies which can be classified into three general categories: soft films, conventional hard coatings, and ion/plasma modified hard coatings.
Soft film technologies generally involve deposition of soft, marginally adherent multilayer thin films onto various glasses. The films are soft and lack physical durability; most films are also water soluble. To protect these sensitive multilayer optical coatings, they are often imbedded onto a transparent epoxy by lamination onto other glass substrates. Optical filters made by a soft film deposition include multiple coating layers and laminations, requiring a burdensome and relatively costly manufacturing process. Moreover, the epoxy laminate effectively limits the useful temperature range of the product (ca. &lt;100.degree. C.), and the epoxy can discolor and degrade when exposed to ultraviolet radiation. Still further, soft film filters can be sensitive to temperature and humidity and therefore have relatively limited operable lifetimes.
Conventional hard coating PVD usually consists of electron-beam evaporated oxides (e.g., TiO.sub.2, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, etc.) layered onto heated substrates (e.g., 250.degree.-300.degree. C.). The deposited films are typically columnar and porous in micro structure. The optical behavior of multiple layers applied by conventional hard coating is not ideal; the refractive indices of evaporated thin films are generally lower than those of corresponding bulk materials. Additionally, conventional hard coatings can absorb and desorb water vapor because of their porous nature. As a result, the spectral behavior of such optical filters can dramatically change and drift, depending upon the environmental exposure. This spectral instability effectively limits the use of such filters for many applications.
In order to circumvent such shortcomings, filters produced by conventional hard coating PVD processes have been encapsulated by laminations to other glass substrates. Such a lamination procedure, however, poses some notable problems. Lamination is costly, can limit the filters to a planar shape and can sacrifice image quality. Another approach has been to use the filters in environmentally controlled housings. This procedure however is also costly and can make use of the filters more burdensome.
Evaporative, ion-assisted deposition processes also have been employed. An ion-assisted process utilizes an ion gun directed toward a substrate within an evaporation coating chamber. When energized, the ion gun directs high-energy particles directly toward the substrate where they collide with previously vaporized coating material that has deposited on the substrate surface. This bombarding action compacts the coating material and creates a denser film structure. However, coatings applied by such ion-assisted methods can eventually spectrally shift over extended periods of time. Additionally, the bombarding high-energy particles tend to become incorporated into the coating layer which may degrade the coating quality and adversely effect the coating density. This process is also difficult to optimize in terms of uniformly creating homogeneous, dense films over large areas, which is needed for high yield, high volume production.
Ion plating processes also have been used to apply coatings. In general, an ion plating deposition process employs a plasma-supported reactive evaporation of a coating material in a vacuum, particularly by vaporizing the coating material by means of an electron beam under reduced pressure. During the process, substrates obtain a negative electrical charge. The vaporized coating material is in the form of positively charged ions. This coating material is directed to and then condensed on the targeted substrate. The high energy of this condensate (e.g., on the order of 20 to 100 eV) is due to electromagnetic attraction between the ionized coating materials and the negatively biased substrates. This is distinct from an ion-assisted process where the energized ions from an electron gun are directed toward the substrate and merely compact the previously vaporized, deposited coating material. See, for example, the processes and apparatus reported in U.S. Pat. Nos. 4,333,962, 4,448,802, 4,619,748, 5,211,759 and 5,229,570. Coating films applied by an ion plating deposition process are completely dense and do not spectrally shift upon exposure to varying temperature and humidity conditions.
Certain coating materials, including low and high refractive index materials in combination, have been deposited to produce optical interference filters. Low refractive index (nL) materials include e.g. SiO.sub.2, Al.sub.2 O.sub.3, SiO, fluorides such as barium fluoride and lanthanum fluoride, MgO, etc. Collectively, low-index materials are sometimes referred to herein as "L". Low-index materials (L) are defined to mean herein materials having a refractive index (20/D) of less than 2.0, more typically 1.8 or less such as 1.8 to 1.3. Common high refractive index (n.sub.H) materials include e.g. TiO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, and HfO.sub.2. Collectively, these high-index materials are sometimes referred to herein as "H". High-index materials (L) are defined to mean herein materials having a refractive index (20/D) of 2.0 or greater. As known to those skilled in the art, the designation "20/D" indicates the refractive index values are as measured at 20.degree. C. using a light source of the D line of sodium.
Ion plated optical interference coatings are often produced by successive deposition of alternating layers of L and H thin films (each film layer typically tens of nanometers thickness for visible wavelength optical performance). Coating layers having alternating L and H thin films are sometimes designated herein as L/H coatings or other similar designation. A common such multilayer combination has been SiO.sub.2 (a low index material) +Ta.sub.2 O.sub.5 (a high index material). Procedures for depositing the alternating layers have included use of an ion plating coating apparatus that has two separate electron beam guns, with one electron beam directed to a containment vessel containing the L layer reagent (e.g., a Si reagent) and a second electron beam gun directed to a containment vessel containing the H layer reagent (e.g., tantalum). The two electron beam guns are alternatively operated without interruption to thereby apply alternating L and H layers, e.g., alternating SiO.sub.2 and Ta.sub.2 O.sub.5 layers.
However, it has been found that ion plated L/H multilayer coatings of many materials (including e.g. Ta.sub.2 O.sub.5 /SiO.sub.2 multilayer coatings) exhibit physical properties that are undesirable for many critical applications. In particular, ion plated L and H multilayer coatings often exhibit significant compressive coating stress which can lead to physical deformation of the underlying substrate. A deformed substrate can also interfere with optical properties of the applied coating layer, particularly image resolution.
Certain other ion plated L/H multilayer coatings have exhibited substantial optical absorbance rendering the coatings of little or no use for many applications. This absorbance problem has been particularly severe for multilayer coatings that contain layers of titanium oxide. In K. H. Guenther, Optical Thin Films and Applications, 1270: 211-221 (August 1990), it is reported that the measured spectral transmittance of a multilayer stack of SiO.sub.2 and titanium oxide is 28% lower than theoretical transmittance. See FIG. 11 of the Drawings which graphically depicts those results reported by K. H. Guenther.
It thus would be desirable to have improved multilayer L/H-type ion plated coatings, including such coatings that exhibit low optical absorbance and low stress. It would be particularly desirable to have improved highly transparent ion plated coatings that contain layers of the high refractive index material titanium oxide (especially TiO.sub.x, x.apprxeq.2). It would be especially desirable to have highly transparent multilayer ion plated coatings that contain layers of titanium oxide and silicon dioxide.